1. Field of the Invention
The present invention relates to a process for the production of a coating on a porous and/or absorbent material by the radiation-induced polymerization of a liquid coating composition.
2. Description of the Prior Art
Coating compositions based on low-viscosity esters of (meth)-acrylic acid, which can be hardened by radiation, are known for example from DE-A 37 06 355. Such coating compositions with a dynamic viscosity typically lower than 1000 mPa.s are applied without the use of solvents and with the addition of photoinitiators and optionally additives, by various process, such as rolling, to substrates to be coated and then hardened by the action of UV radiation.
The low viscosity of the coating composition, which is important for applying thin layers, is a disadvantage if the substrate to be coated has porous and/or absorbent properties. Then, the liquid, low-viscosity coating composition can penetrate the substrate, with the result that when it is subsequently irradiated, the parts of the coating composition which have penetrated are not, or not fully, reached by the radiation and, thus, are not, or not completely, hardened. The unhardened, liquid components then have a detrimental affect on the quality of the coating. A phenomenon, among others, known to the person skilled in the art as ‘sweating out’ then occurs, in which liquid components seep through the hardened film and become visible on the surface of the coating. This effect can occur even after a very short time, for example a few hours, or only after a longer time, for example a few months.
An object of the present invention is to provide a process for producing a coating on a porous or absorbent material by applying the coating composition and then hardening it by UV radiation, which prevents sweating out of unhardened components.
This object may be achieved by the addition to the coating composition of certain additives, commonly used as thixotroping agents, which belong to the group of polyamides or oligomeric or polymeric fatty acid amides.